1. Field of the Invention
The present invention relates to an automatic frequency control method wherein a frequency value of a voltage controlled oscillator is increased or decreased and stored by determining whether an oscillation control voltage of the voltage controlled oscillator is in a range capable of operating a phase locked loop when an oscillation frequency of the voltage controlled oscillator of the phase locked loop is not synchronous with the intermediate frequency when a frequency is automatically controlled by comparing phases and frequencies of the phase locked loop and the intermediate frequency.
2. Prior Art
In general, when a television and a video cassette tape recorder, which process image and sound signals, selects a channel, a control section, such as a microcomputer, compares the intermediate frequency IF with an oscillation frequency of a voltage controlled oscillator of a phase locked loop PLL for an automatic frequency control, and controls reception of a desired channel. As shown in FIG. 4, an automatic frequency control apparatus comprises a reception section 10 for receiving a radio frequency signal (RF signal) input through an antenna, detecting a level of the RF signal and controlling the RF signal to become an exact signal frequency; an intermediate frequency amplification section 11 for amplifying an output signal of the reception section 10 up to desired voltage level; a demodulation section 12 for outputting an image signal and a sound signal from an output signal of the intermediate frequency amplification section 11; a phase locked loop 13 for maintaining a certain frequency; and comparing the intermediate frequency with the oscillation frequency of the voltage controlled oscillator and a control section 14 for increasing and decreasing a frequency value of the voltage controlled oscillator based on the comparison result.
In the automatic frequency control having a construction as shown in FIG. 4, the reception section 10 receives a RF signal input from an antenna. The received RF signal is input to the intermediate frequency amplification section 11 which outputs a desired signal component by controlling the RF signal to become an exact signal frequency according to a frequency value of the voltage controlled oscillator which is fed from the control section 14. A signal input to the intermediate frequency amplification section 11 is amplified by a desired voltage level for an output.
Further, the phase locked loop 13 compares the intermediate frequency with a phase and a frequency of an oscillation signal of the voltage controlled oscillator and outputs a comparison result to the control section 14. The control section 14 increases or decreases a frequency value of the voltage controlled oscillator according to a value output from the phase locked loop 13. The increased or decreased frequency value is fed to the reception section 10 as an exactly controlled frequency. Such an exactly controlled frequency is input to the demodulation section 12 through the intermediate frequency amplification section 11 to output an image signal and a sound signal.
FIG. 2 is a flowchart that shows a conventional automatic frequency control method in an automatic frequency control apparatus. As shown in FIG. 2, first a factory mode is set (S10), then a voltage controlled oscillator adjustment mode is selected (S11). Thereafter, the value of the uppermost bit AFA of an inner interface control bus which is transmitted from the phase locked loop 13 to the control section 14 is checked (S12). The value of the uppermost bit AFA becomes "0" when there is a long difference between the intermediate frequency and the frequency of the voltage controlled oscillator of the phase locked loop 13. The value of the uppermost bit AFA becomes "1" when the difference between the intermediate frequency and the frequency of the oscillation signal is less than a predetermined value.
In step S12, when the value of the uppermost bit AFA becomes "0", the value of the lowermost bit AFB of the inner interface control bus, which is transmitted from the phase locked loop 13 to the control section 14, is checked (S13). In step S12, when the value of the lowermost bit AFB is "0", a frequency value of the voltage controlled oscillator is increased to synchronize the intermediate frequency with an oscillation frequency of the phase locked loop 13(S14). When the value of the lowermost bit AFB is "1", the frequency value of the voltage controlled oscillator is decreased (S15). Thereafter, the procedure returns to step S12 and this step is repeated until the difference becomes less than a predetermined value.
In step S12, when the uppermost bit AFA is "1", the lowermost bit AFB of the inner interface control bus is checked(S16). If the lowermost bit is "0", the frequency value of the voltage controlled oscillator is increased (S17). If the lowermost bit is "1", the frequency value of the voltage controlled oscillator is decreased(S18).
Further, a determination is made as to whether the value of the lowermost bit AFB is changed from "0" to "1" or vice versa (S19). When the value is changed, variation of the voltage controlled oscillator stops and the frequency value of the voltage controlled oscillator is stored because the oscillation signal of the phase locked loop 13 is synchronized with the intermediate frequency(S20). When the value is not changed, a value of a counter N is increased(S21) and it is determined whether the value of the counter N is larger than a predetermined value(S22). In the above step S22, if the value of the counter N is larger than the predetermined value, an error message is displayed (S23) and the procedure ends. If the value of the counter N is not larger than the predetermined value, the procedure returns to step S16 and the subsequent steps are repeated.
Such a conventional automatic frequency control method compares the intermediate frequency with the oscillation frequency of the voltage controlled oscillator of the phase locked loop and controls a frequency by means of status changes of the uppermost bit and the lowermost bit according to the difference between the two frequencies. Therefore, the conventional automatic frequency control method has a drawback in that a frequency is not controlled under certain circumstances.